Aragonite saves ages from landslides

New method of dating developed

Aragonite (chemical: CaCO3), which formed immediately after the Fernpaß landslide in the landslide mass. This aragonite was used to determine the proxy age of the landslide using the U / Th method. © University of Innsbruck
Read out

Landslides are normal in the Alps - today and in the past. But when exactly did the respective historic landslide take place? So far geologists have been unable to give an exact answer to this question. Now scientists have developed a new method that can accurately determine the age of landslides.

Scientists have long suspected a correlation between climate variability and the frequency and size of landslides. Above all, an increase in precipitation and the melting of glaciers could lead to such events occurring more frequently. In order to better understand this relationship, the most accurate determination possible of the age of landslides is an indispensable prerequisite.

The Fernpass landslide was first recognized by the ice age researcher Albrecht Penck in the late 19th century as such. Since then, this landslide, which is the third largest of the Eastern Alps with about one cubic kilometer mass, was frequently studied. Larger landslides are - as long as they go down to the valley - extremely mobile masses that race downwards at speeds of up to several hundred kilometers per hour. As part of a research project at the "alpS Center for Natural Hazards Management GmbH", in which the University of Innsbruck was decisively involved, the Fernpass landslide with explosive seismic and georadar measurements was re-explored. The results showed that the landslide mass at the Fernpass crest is several hundred meters thick.

Findings so far too inaccurate

While many aspects of landslides have been known since the 19th century, determining their ages is in most cases much more difficult. "The problem is that" conventional "geological findings can hardly be used to determine age, " says Professor Diethard Sanders from LFU Innsbruck.

The previous approach to age determination of landslides was to use the radiocarbon method to determine the age of wood remnants deposited in lakes dammed by the landslide. Unfortunately, this method often does not lead to satisfactorily accurate or sufficiently safe results. The case was similar at the Fernpass landslide. Over time, different geologists have proposed very different ages, but none of the conventional geological findings were unambiguous. display

International cooperation brought solutions

Taking into account detailed field investigations by Christoph Prager (alpS, University of Innsbruck), the Fernpass landslide has now become possible for the first time thanks to the cooperation of geologists, geochemists and physicists from various research institutes at the University of Innsbruck, Uni Z rich and Uni Bern very narrow age range.

The solution to the problem was two different measuring methods. The physicist Susan Ivy-Ochs from the University of Zurich has dated the exposure age of the rock surface of the huge eruption niche of the landslide with the help of the chemical changes in the rock, which are caused by cosmic radiation. The two ages obtained indicate how long ago a rock face was uncovered by demolition: 3600 900 years; 4800 1100 years; Average 4100 1300 years, the most likely age of landslide.

Unique Innsbrucker method discovered

Geologists from the University of Innsbruck and from alpS took a completely different route. We found something that had never been noticed before: Under huge landslide blocks it was just after the landslide to the chemical segregation of a certain type of lime, the so-called "aragonite" came, "says Sanders.

The aragonite from the Fernpass contains the radioactive elements uranium and thorium. The isotope uranium decays over time with the same probability in the isotope 230Thorium. Sanders goes on to explain so you have a "clock" that allows you to measure the time since aragonite formation by measuring the relative amounts of thorium and uranium in that mineral!

As part of a Sanders-led project, Marc Ostermann's dissertation on the Uranium-Thorium method was able to determine the age of the landslide with unprecedented accuracy. The measurements of Ostermann were carried out at the University of Bern, in the laboratory and under the guidance of Professor Jan Kramers. Accordingly, the landslide went down at least 4, 150 100 years ago today. The age of the Fernpass landslide, which was determined using two completely different methods, is impressive (4, 100 1, 300 years: rock exposure age average; 4, 150 100 years: U / th age) aragonite).

Prerequisite for making better predictions

The "Innsbruck Method", to limit the age of a landslide by means of the uranium and thorium contents of newly formed lime sinter, was used for the first time worldwide in the Fernpass landslide. Continuing investigations have shown that similar calcareous sinters have formed again in other rockfalls. Therefore, the lime sinter from Fernpass landslide are by no means an exception.

The rapid and comparatively cheap uranium-thorium method is to be used in future also in other mountain ranges of the alps, in order to determine their age more exactly. "This is an essential prerequisite for the development of improved predictive models of the relationship between climate or global change and possible natural hazards of the future in the Alpine region", says Sanders.

(University of Innsbruck, 16.03.2007 - DLO)